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Scholars have argued that childhood experiences strongly impact political attitudes, but we actually have little causal evidence since external factors that could influence preferences are correlated with the household environment. We utilize a younger sibling’s gender to isolate random variation in the childhood environment and thereby provide unique evidence of political socialization. Having sisters causes young men to be more likely to express conservative viewpoints with regards to gender roles and to identify as Republicans. We demonstrate these results in two panel surveys conducted decades apart: the Political Socialization Panel (PSP) and the National Longitudinal Survey of Youth (NLSY). We also use data collected during childhood to uncover evidence for a potential underlying mechanism: families with more female children are more likely to reinforce traditional gender roles. The results demonstrate that previously understudied childhood experiences can have important causal effects on political attitude formation.